The present invention relates generally to magnetic bearing designs that incorporate high-temperature superconductors which are useful for flywheel energy storage. More particularly, the present invention relates to magnetic bearing designs utilizing a combination of permanent magnets and high temperature superconductors. The bearing designs are capable of large levitation pressure, low rotational loss, and the use of a wide variety of high temperature superconductors. An important feature in each of the designs is that a permanent magnet configuration is used to provide most of the levitation force, preferably in a geometry that is close to neutral equilibrium, so that the stiffnesses associated with this part of the bearing are small. The high temperature superconductor part of the bearing provides high stiffness but not necessarily high levitation pressure. When these bearings are used in flywheel energy storage devices, the efficiency of the flywheel can be very high and flywheels of this type become economic for diurnal energy storage and other applications where high energy efficiency is important. Further information relating to flywheel energy storage devices can be found in copending patent applications assigned to Argonne National Laboratory entitled "Optimization of Superconducting Tiling Pattern for Superconducting Bearings" and "Improved Permanent Magnet Design For High-Speed Superconducting Bearings."
Storage of electrical energy is useful in a number of applications. Diurnal storage of electricity is important to electric utilities in order to efficiently utilize base load generating plants and to meet the varying load demands of their customers. In this example, the baseload plants can charge the storage units at night when demand is low, and then peak demands can be met by discharging the storage units during the peak hours.
Energy storage can also play a substantial role in eliminating or postponing the installation of larger capacity power lines. Power can be transmitted at night to a substation or user energy storage unit when demand is low; and then during peak power times, the energy storage units can be discharged. The placement of energy storage units can occur in various parts of the electrical distribution system: utility parks where large amounts of energy can be stored; in tandem with photovoltaic or wind energy generation facilities that are time dependent; substation units; and individual companies and houses. Other potential applications for energy storage include use in electric vehicles such as cars and buses, and wayside energy storage for electric trains.
Flywheels are often considered for energy storage applications. Their primary advantages are modularity, high energy storage density (Wh/kg), and high efficiency input and output of electrical energy. The ability to produce high strength conventional flywheel rotors and the ability to efficiently transfer energy in and out of a flywheel are well known and will not be discussed herein.
The primary disadvantage of conventional flywheels is inefficiency in a standby mode. This inefficiency is caused by high rotational losses of the bearings that support the flywheel structure. High temperature superconductor bearings constructed in accordance with one form of the present invention substantially reduce rotational losses and enable standby losses in flywheels to be 0.1%/hr or less. These high temperature superconductor bearing designs provide passive stability in all directions, i.e., they provide a positive stiffness in all displacement directions. These designs also allow rotational motion with very low friction.
One of the difficulties with high temperature superconductor bearings is that the levitation pressure derived from structures obtained by most processing methods is relatively low. To achieve high levitation pressure, expensive processing methods must be used that enable large crystals of high temperature superconductors to be grown. For the same critical current density, the magnetization of a high temperature superconductor is proportional to the linear size of the crystal. From investigations of different high temperature superconductor materials, it was discovered that magnetic stiffness in high temperature superconductor materials is substantially independent of crystal size if the amplitude over which the stiffness is calculated is relatively small, e.g., of the order of a millimeter. Thus under these conditions, if the high temperature superconductor is used primarily for stiffness purposes, less expensive processing methods can be used.
In further experiments, it was determined that the rotational loss between a permanent magnet and a high temperature superconductor decreases as the levitation height increases. Since these bearings are often cooled while in use in the field, there is some range of choice in the levitation height. However, large levitation pressures require low levitation heights to be realized. Because relatively low levitation pressures are required by the preferred embodiments of the invention, rotational losses can be reduced by increasing the levitation height of the permanent magnet over the high temperature superconductor.
Gas centrifuge designs are well known, and these designs often employ magnetic bearings (see S. Whitlow, "Review of the gas centrifuge until 1962," Rev. Mod. Phys., Vol. 56, pp. 67-97, 1964.) To overcome the inherent instability in these bearings, a pivot bearing is often employed as shown in FIG. 1A. In this design, the magnetic bearing provides most of the levitational force but is axially unstable while being radially stable. The mechanical pivot bearing at the bottom provides both axial and some additional radial stability. This design locates the rotor close to the support so that the magnetic bearing clearance is small, but not so close that a minor perturbation will cause the rotor to jump up and attach to the magnet. This pivot is very stiff; however, in high-speed machinery applications, the bearing wears and provides a source of frictional loss.
These disadvantages have prevented diurnal energy storage and other high energy efficiency applications using superconducting bearings from being economically feasible.
It is therefore an object of the invention to provide a novel low-loss bearing and method of use that can be used to achieve a high-efficiency flywheel energy storage device.
It is a further object of the invention to provide an improved passively stable bearing and method of use in which a substantial portion of the levitation force is carried by the interaction of two or more permanent magnets while a substantial portion of stabilization forces are provided by a high temperature superconductor structure.
It is a still further object of the invention to provide low drag force in a novel magnetic bearing comprising a permanent magnet and a high-temperature superconductor.
It is yet another object of the invention to provide an improved magnetic bearing and method of use having reduced rotational losses.
It is a further object of the invention to provide a novel magnetic bearing and method of use including a high temperature superconductor structure located a relatively large distance away from a rotor to provide stabilization forces while reducing rotational losses.
It is a still further object of the invention to provide an improved superconducting magnetic bearing and method of use including a combination of permanent magnets pushing and pulling on a rotor structure to provide levitation force.
It is yet another object of the invention to provide a novel superconducting magnetic bearing and method of use using a superconductor structure exclusively as a passive stabilizer located at a distance away from a rotor sufficient to provide stabilization forces on the rotor without levitating the rotor.
It is another object of the invention to provide an improved superconducting magnetic bearing and method of use which reduce rotational losses due to magnetic field inhomogeneity.
Other advantages and features of the invention, together with the organization and the manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the drawings.